In a time-of-flight mass spectrometer, ions are extracted from a source containing a product to be analyzed and their mass is determined by measuring their time of flight up to a detection device.
The source of ions is for example constituted by a solid surface from which the ions are released by desorption. Several techniques are used to that end. For example, the solid surface may be bombarded with primary ions accelerated by cyclotron or be subjected to a high-energy radiation. It is also well known to employ a radioactive source .sup.252 Cf which emits two fission fragments in opposite directions, one being directed towards the solid surface to release ions and the other towards a metal foil to eject electrons of which detection furnishes the time reference (starting signal).